How can i join mesh for sculpting
There are two problems with your meshes that is preventing the boolean from working properly.


The normals are reversed on the lower object in relation to how they are set for the upper object. This is what is causing the object to disappear after the boolean. Reverse the normals on the lower object by going into Edit Mode for that object and pressing Shift + n to reverse them.
Each of your objects has edges and faces pinched in certain areas to the point that they are crossing through one another. This also must be changed, or else the boolean will register as successful, but won't actually combine your shapes into one. I fixed the situation by using a smooth modifier on each. But it wasn't quite enough to completely straighten out the faces, so I lightly added a few short strokes in Sclupt Mode with the smooth brush, and all was good.


The included file shows both what the new version of these shapes look like after being tweaked but before being boolean unioned, as well as what they look like after the boolean.